plantconc - All plants have to transpire, have water...

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All plants have to transpire, have water evaporate from leaves. When water evaporates from leaves, the plant has to restore the lost water. It can do this in many ways using the push and pull mechanisms of xylem. The push mechanism is caused by nutrients flowing into the vascular cylinder (where xylem is located) in the roots from the soil, getting to cylinder by either apoplastic, symplastic, or transmembrane (from apoplastic to symplastic travel). Nutrients that flow into the xylem of the vascular cylinder are making the water potential of the dead xylem cells negative, therefore causing osmosis, diffusion of water into cells, to occur, sometimes with aid of aquaporins. The water flowing into the xylem cells causes a turgor pressure down in the root, which pushes water up to the leaves to replace lost water. The other pull mechanism happens more commonly than the push mechanism and is used for bulk flow. In this mechanism, transpired water molecules are replaced by the molecule in the water vapor in the air spaces between mesophyll cells of the spongy parenchyma. After all this water is taken, water is extracted from a water membrane interface that borders the cell walls of the mesophyll. As water is taken from this interface, pressure is taken off the cells, and the interface starts to curve more. This is negative pressure and is called so because it results in a negative water potential in the
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plantconc - All plants have to transpire, have water...

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